Lubrication-free centrifugal compressor

ABSTRACT

A gas compression compact device comprised of: a) one or more centrifugal compressors; and b) a high speed axial flow permanent magnet synchronous electric motor. The electric motor and the compressor are directly coupled on a single axis and supported by passive magnetic and electrodynamic bearings, free of lubrication. The equipment does not use mechanic seals since the rotor is placed inside the pressure containment of the gas. The equipment does not require auxiliary systems for cooling, filtration, separation or feeding of lubricant fluids.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. provisional application No. 62/769,323, filed 19 Nov. 2018, which is hereby incorporated by reference as though fully set forth herein.

FIELD OF THE INVENTION

The present invention pertains to the field of gas compression equipment. In particular, the present invention refers to compact electric devices or equipment which use centrifugal compressors to increase a gas pressure.

BACKGROUND

Gas compression by mechanical means is currently carried out through different technologies which are widely spread and tested from some years ago. Among the most used methods are centrifugal, axial, alternative and screw compressors.

Alternative, or piston, compressors are similar in construction to internal combustion engines. A piston or plunger displaces longitudinally in an alternate manner inside a cylinder, against which a series of seals are located. A set of valves allows the entrance of gas during the expansion phase of the chamber formed by the cylinder and the plunger, while the latter gets back. When it advances again, the plunger reduces the volume of the chamber where the gas is located, thus producing the subsequent desired pressure increase. Finally, another set of valves allows the escape of gas at high pressure. These compressors are widely used in small and medium scale applications. They are of simple construction and their efficiency is relatively high. They have numerous mobile components but they move at low speed relative to each other, therefore, they can use traditional lubricants for their seals and bearings. By functioning at low speed (around 1,000 to 5,000 rpm) they usually couple to an electric motor or combustion engine in a direct manner, with no gear assemblies.

The main drawback of this technology lays in the intensive maintenance required by the lubrication system, which is needed to assure the sealing between plungers and cylinders, the integrity and water tightness of valves and to avoid damage in bearings. With a proper maintenance, alternative compressors may function with no inconveniences for many years. However, all the above mentioned components suffer from mechanical wear and should be periodically replaced, even when they are maintained in optimal lubricant conditions. Due to the number of mechanisms involved, alternative compressors are the most bulky of the four types mentioned (with low specific power).

Screw compressors, as well as the alternative ones, belong to a group of machines called “positive displacement machines”, i.e., the change in gas pressure is achieved from a change in the volume of the chamber containing it. In a screw compressor, two screw-shaped helical axes, located parallel between each other, turn in a unified manner, one against the other. Both helicoids are within a chamber which has walls that are very close to the edge of their threads, thus forming a mechanical seal with or without help of lubricants. The turning of screws against the chamber generates hermetically isolated volumes containing gas and which progressively reduce their size while they displace from the suction port to the discharge one.

This type of compressor is typically used in medium scale applications and shows some advantages such as a reduced size and high compression rates. The mechanical components turn at moderate speeds (for example, between 3,000 and 10,000 rpm), therefore, they may require gear assemblies to couple to electric motor or combustion engines, which are typically slower. In opposition, they have fewer mobile components than the alternative compressors; therefore, the mechanical wear is less relevant as regards maintenance. The major drawback lays in the lubrication system requirements, which should be maintained free of solid particles and should be constantly cooled. The lubricant plays a key role by acting as a seal between the helicoids, and as the means to transmit torque between them with no mechanical friction. Additionally, the lubricant present in the compression chamber is mixed with the compressed gas (especially if they are both similar fluids, such as hydrocarbons) and should be separated and recycled in a set of specific devices. For these reasons, the reduced size of a screw compressor is offset by the high number of auxiliary systems accompanying it.

Centrifugal compressors belong to the group of turbo machines along with axial compressors, turbines and centrifugal pumps. In a centrifugal compressor, the increase in gas pressure is achieved indirectly, by first increasing the speed of the gas and then converting this kinetic energy into potential energy. A disk with blades called an impeller receives gas at low pressure and rotationally accelerates it, at the same time it displaces it to the periphery. When the high speed gas exits the rotor, another fixed component called a diffuser is in charge of gradually deaccelerating it, thus increasing its pressure.

Centrifugal compressors show some important advantages over the positive displacement technologies: First, for the same application (flow rate and pressure) their size is much smaller than an alternative or a screw compressor (they have high specific power). Second, since they do not use seals to generate waterproof chambers, the internal components, such as the impeller, do not suffer from mechanical wear. The function of the lubricant used in these compressors is to reduce the friction on the auxiliary components such as external seals and/or bearings.

The problems faced by these compressors are related to the high speed at which their impellers should spin (typically between 20,000 and 100,000 rpm). Therefore, there is a need to use gear assemblies to multiply the speed of driving sources such as electric motor or combustion engines. Gear assemblies are a key component, both due to the mechanical wear and the loss of energy they imply. Another problem appears on the seals that keep the process gas isolated from the atmosphere. Said seals allow the communication between the rotor axis and the gear assembly, thus keeping the high gas pressure inside the compressor body and low pressure in the outside. The seals are a key component as regards wear, in which lubrication plays a key role.

The above mentioned complexities have limited the use of centrifugal compressors to large scale applications. However, centrifugal compressors that are suitable for medium and small scale have recently been developed thanks to the implementation of such technologies as high speed electric engines. The firm Aerzen®, for example, offers air compressors in which a radial flow electric motor is mounted on the same axis as the rotor of the compressor. This way, the need of a gear assembly and its associated operating difficulties are eliminated.

The main disadvantage appearing on the use of high speed radial flow motors lays on the great amount of ferromagnetic material present on stators. The high electric frequency used at high speed implies huge losses of energy in said cores as parasitic currents and magnetic hysteresis. An alternative to this technology is that of axial flow electric motors, which has gained great interest over recent years due to a series of advantageous constructive features.

First, the geometry of axial flow motors allows obtaining machines with higher specific power than radial flow motors. That is; in order to develop the same power, an axial flow motor requires a smaller size than its radial flow equivalent, with the subsequent reduction in weight and cost. As well as the radial flow motors, those of axial flow may be either inductive and use winded conductors equivalent to a “squirrel cage” or synchronous and use permanent magnets. The latter show the highest power density of all possible configurations.

Second, and with more relevance for the application of this invention, the arrangement of coils in an axial flow motor requires the use of much less ferromagnetic material in their core than in a radial flow motor. In high electric frequency applications (high spinning speed) a reduction of the ferromagnetic material implies a decrease in energy losses due to magnetic hysteresis and parasitic currents, therefore, significantly increasing the performance of the machine and reducing the cooling requirements. The industrial use of axial flow synchronous motors in high speed applications, which is not very well known in current art, promises important improvements as regards size reduction and energetic efficiency increase.

The firm ICR Turbine Corporation® (Patent Publication No. US20140306460) has developed a compact Brayton cycle implementing several compression and expansion stages to increase the global performance of the system and to boost a radial flow electric generator coupled to an independent turbine. Each one of the compression and expansion stages comprises a radial compressor and a radial turbine, both mounted on a single axis. At the back of each radial compressor a small axial flow motor has been mounted that allows for the starting of the system, thus driving the rotor to its minimum holding speed. These engines are the inductive type and comprise a winded stator and a planar squirrel cage rotor.

Additionally, several technologies have been developed to support the motor-compressor rotating assembly with no need to use lubricating oils, thus eliminating another complexity associated with the high speed of these machines. One of them is known as “air foil bearing” or simply “foil bearing” in which the same process gas is used as lubricating fluid. The foil bearings are typically used in air compressors, such as the ones from Aerzen®.

Another more promising technology is the one of magnetic bearings. In this case, the rotating set is supported by the magnetic fields and not hydrodynamically as in the prior case, thus making the system independent of the process fluid. The firm Danfoss®, for example, offers the “Turbocor®” compressors for cooling gases in which the rotor is suspended by active magnetic bearings (AMB). The engine-compressor assembly is located in a water tight chamber containing the process gas, this way also eliminating the need for mechanic seals. Another example of this technology is found in the firm SIEMENS® (Patent ES2309173) which has developed a large scale centrifugal compressor, in which the engine and the compressor are mounted on the same axis and housed in the same pressure containment. In the case of SIEMENS® the bearings are also of the AMB type and also the same process gas is used to cool the motor.

The complexity shown by the AMB lays in the need for an axis position sensor set and an active control electronic system which externally energizes and commands the bearings. In the event of an energy interruption of the control system, the rotating axis may lose support and contact the stator spinning at high speed and producing significant and permanent damage.

SUMMARY OF THE INVENTION

The present invention comprises a compact sized centrifugal compressor, for medium and small scale applications, which does not use lubrication systems, speed multiplication systems or mechanical seals and which works usually at very high spinning speeds.

This compressor uses, as the driving source, an axial flow and permanent magnet electric motor operating synchronically at high speed and high electric frequency. Said motor operates efficiently at high speed, since it has less ferromagnetic core in its stator than the equivalent radial flow motors. In fact, this motor may work efficiently even with no ferromagnetic core in its stator. The compressor impeller is directly coupled to the motor on a single axis, forming a single mobile part of the device. The high efficiency of the axial flow motor and the absence of speed multiplying gears give this invention higher reliability and global energy efficiency over the current art.

The motor-impeller rotating assembly is supported by a combination of magnetic radial bearings and electrodynamic thrust (or axial) bearings, such that there is no mechanical contact with the stator. Said bearings use permanent passive magnets, therefore they do not require control systems, sensors or external energy supply. This gives the invention a higher operating simplicity and reliability than the one of the current art.

The complete rotor, formed by the motor-impeller assembly and the magnetic and electrodynamic bearings, is located inside a waterproof chamber pressostatically linked to the stator of the compressor and flooded by the same process gas. In this way the use of mechanic seals is avoided as well as the subsequent wear by friction, which gives this invention the feature of requiring less maintenance than other similar current art equipment.

By not requiring lubrication for bearings, gears or seals, the device of this invention does not use auxiliary systems for cooling, filtering, separation or feeding of lubricants. This allows the invention for higher simplicity and smaller size than other similar devices of the current art.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a side view of an embodiment of the invention which shows only the motor-impeller rotating assembly and the stator coils of the motor.

FIG. 2a is a cross-sectional side view of the main components of the axial flow motor.

FIG. 2b is an exploded perspective view of the main components of the axial flow motor.

FIG. 3 is a cross-sectional view of an embodiment of the invention including all the components of the device.

FIG. 4 is a full perspective view of an embodiment of the device and two orthogonal views of same, being the latter compared to the silhouette of an average adult for dimensional reference.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a compact device for gas compression driven by an axial flow synchronous electric motor with no use of any kind of lubricants.

FIG. 1 shows the single mobile piece of the motor-impeller rotating assembly (hereinafter, the rotor). Said rotor has a centrifugal impeller 1 in charge of delivering kinetic energy to the process gas. In the embodiment of FIG. 1 only one impeller is shown but more than one may be used. Said impeller is mounted on an axis 5 and fixed thereto. If more than one impeller is used, all of them may be mounted on and fixed to the same axis.

An axial flow synchronous and permanent magnet electric motor 2 is formed by a stationary stator section and a rotating section. The stator section of said motor contains the coils, different auxiliary pieces for support and may optionally contain part of the ferromagnetic core. This section may be formed by one or more assemblies located between assemblies of a rotating section. In the embodiment of FIG. 1, two stator assemblies 4 are shown located between rotating assemblies 03. In other embodiments this could be applied to more than two stator assemblies or only one may be used.

FIGS. 2a and 2b show an embodiment of the axial electric motor in which it is comprised of a single stator assembly 4. Said assembly contains coils 10 and different portions of the ferromagnetic core 11. The assembly 4 is placed between two rotating disks 3 that contain the permanent magnets 12 and may optionally contain another part of the ferromagnetic core 13. These discs form the rotating section 3 of the axial flow motor. Said rotating section is mounted onto the axis 5 (see FIG. 1) and fixed thereto.

The magnetic flow is established between each opposing pair of permanent magnets 12, which are placed in an attraction configuration. Said magnetic flow passes through the coils through air or any other means in which the motor is immersed. If the stator assembly contains portions of ferromagnetic core 11, the magnetic flow is concentrated through these. If a rotor assembly 3 has a ferromagnetic core 13, the flow between opposite faces of its adjacent magnets is closed therethrough. An external electronic device monitors the relative position of magnets 12 as regards the coils 10 and activates a series of semiconductors (for example: mosfet, IGBT, SSR, etc.) that inject current to the latter. The moment and the duration of current pulses is such that their interaction with the magnetic field induces a force over the permanent magnets resulting in a torque applied onto the axis 5 (see FIG. 1). The stator section of the motor 04 may contain one or more coils 10 either electrically independent or linked to each other.

In FIG. 1 it may be seen that the rotating section of a first magnetic radial bearing 6 is formed by one or more permanent magnets with ring geometry and is mounted on one end or near to an end of the axis 5. The rotating section of a second magnetic radial bearing 7 is formed by one or more magnets with ring geometry and is mounted on one the opposite end or near to the end opposite end of the axis 5. Said rotating sections of radial bearings are part of the rotor. For each bearing, a second stator section is formed by permanent magnets with ring or cylinder geometry that circumferentially surround the rotating sections and are fixed to the housing (not shown). Being the magnets of the same polarity and great field intensity, by means of the magnetic interaction between each rotating section and its stator counterpart, magnetic repulsion forces are developed that allow radially supporting the rotor avoiding its mechanical contact with the rest of the device. In the embodiment of FIG. 1 two magnetic radial bearings are shown, one at each end, but three or more bearings could be mounted on different zones of the axis 5 in order to make the rotor support more rigid. Radial bearings 6 and 7 are passive and operate with no intervention of control systems, sensors or external energy sources. The materials for the manufacturing of magnetic bearings based on high intensity passive magnets such as AlNiCo, SmCo, or NdFeB are well known in the art and are available in the market.

The concept in mechanics of rigidity refers to the capacity of an object to resist a deformation or displacement due to external forces. The more rigid the object, the higher force it generates against the same degree of deformation. This concept, typically applied to elastic systems such as springs and bearings, is also frequently used to describe the mechanical properties of active and passive magnetic bearings. When the forces due to the rigidity of the above mentioned object are such that they tend to compensate the deformation or the displacement that origins them, it is said to have negative rigidity. In the case of magnetic bearings, positive rigidity refers to a particular behavior of these in which the forces originated by a displacement tend to increase it, instead of being opposite. It is useful to note the concept of positive rigidity since this will be used below to explain the functioning of some elements of this invention.

The rotor assembly shown in FIG. 1 is supported and stabilized by the passive radial bearings since they give negative rigidity in the radial direction. That is, if axis 5 is displaced laterally (radially), said bearings react by generating an opposite elastic force returning the axis to its central position. However, as stated by the Earnshaw Theorem, this type of bearing gives positive rigidity in the axial direction. That is, if axis 5 is displaced along its axial direction, said bearings react by generating a force in the same direction and attempting to increase the displacement, therefore the problem arises that these bearings tend to push axis 5 out of its position in the axial direction. Thus, the great advantage of these bearings, originated from their completely passive nature, involves the disadvantage of showing a positive axial rigidity, therefore they cannot be used as single rotating links of the assembly. To counteract this effect, an additional mechanism should be implemented to operate by physical principles that are different to the interaction between permanent magnets and that fix the position of axis 5 in the axial direction. After various tests with different types of restriction links in the axial direction, the best results have been obtained by using the electrodynamic thrust bearing.

In FIG. 1 it may be seen that the rotating section of an electrodynamic thrust bearing 8 is formed by two disks having permanent magnets and ferromagnetic cores. Said rotating section is mounted on axis 5 and fixed thereto, thus forming part of the rotor. A solid or perforated conductor disc 9 is located between said rotating discs and forms the stator section of the thrust bearing, linked to the housing (not shown). The relative movement between the magnets of the rotating section and the conductor material of the stator section induces electric currents on the latter that generate repulsion forces against said magnets. In the arrangement shown in FIG. 1, said repulsion forces give negative rigidity in the axial direction. This way, if axis 5 is displaced in the axial direction, the electrodynamic thrust bearing generates a force in the opposite direction attempting to reestablish the original position.

The electrodynamic thrust bearing works in a completely passive manner and does not require auxiliary control systems. However, said functioning only happens if there is relative movement between parts, that is, only if the rotor is spinning. Above a minimum rotation speed, the electrodynamic thrust bearing provides the rotor with enough negative axial rigidity to counteract the positive axial rigidity of magnetic radial bearings. It is possible to arrange the magnet supporting discs 8 as the rotating section and the conductor disc 9 as the static section, or vice versa. In the embodiment of FIG. 1, only one thrust bearing is shown, but two or more could be stacked to provide higher axial rigidity to the rotor. In applications where the rotor is not horizontally oriented, part of or the whole axial component of its weight may be offset by the same positive rigidity of magnetic bearings. This way, the electrodynamic thrust bearing must only provide negative rigidity to the assembly but it is not used to support its weight.

The combination of magnetic radial bearings with electrodynamic thrust bearings allows the rotor to be completely supported in its axial and radial position, above a specific minimum spinning speed, thus avoiding its mechanical contact with the rest of the device. In opposition to active magnetic bearings (AMB), the combination of passive components in this invention assures its functioning with no external energy or control requirements, even with total interruptions of electric supply. This novel combination allows the device to spin at the required speed by the impeller of the centrifugal compressor without suffering any wear, due to the absence of friction force that would generate a great amount of caloric and stopping energy.

FIG. 3 shows an embodiment of the invention in which the above mentioned rotor is positioned horizontally and allocated inside the body forming the fixed structure of the device (hereinafter, the stator). The orientation of the rotor may be horizontal, vertical or any other orientation different from what is shown in this embodiment. A flanged connection or any other type of connection 15 allows the entering of low pressure process gas to a water tight chamber formed by the main stator walls 16 and other components sealed against them, such as a high pressure collector 17 or an axis end cap 21. The number and arrangement of the components that form said water tight chamber may be different from the ones represented in FIG. 3 but they always contain inside the whole rotor, thus assuring the water tightness character of the gas by means of static seals such as gaskets, O-rings, etc.

The zone where the axial flow motor 2 is located, shown in FIG. 3 as the stacking of 4 stator assemblies and is corresponding rotating sections, is crossed by the low pressure-low temperature gas. This gas flow allows removing the heat generated in the motor, acting as a coolant. Then, the gas enters a first compression stage formed by an impeller 1 and its corresponding diffuser. In the embodiment of FIG. 3, the rotor contains a second impeller 14 immediately crossed by the gas after the first one. At each compression stage the gas increases its pressure and temperature until it enters a high pressure collector 17 and is conducted to a discharge connection of the stator. Other embodiments of the invention may contain more or fewer impellers, as well as more or fewer assemblies forming the axial flow motor, according to each specific application.

In the embodiment shown in FIG. 3, the gas temperature at the entrance is low enough to act as a coolant of the power electronics in charge of commanding the electric motor. A series of power electronic components 19 is placed out of the gas pressure containment 16 and is thermally linked thereto. Said electronic components take advantage of the thermal conductivity of metal forming the pressure containment to be cooled with the same process gas. Finally, a cap 18 externally covers the power electronics to protect it from dust and ambient moisture. Other embodiments of the invention may locate said electronic components directly inside the pressure contention, flooded by the process gas. Other different embodiments may use other conventional and independent methods to cool the power electronics in case the gas temperature at the entrance of the compressor is extremely high.

FIG. 4 shows an isometric view of an embodiment of the invention in which the rotor is oriented horizontally. Said view shows a flanged entrance connection for gas co-axially disposed with the rotor, being the latter out of sight inside the pressure containment. In this embodiment, the high pressure gas discharge is placed laterally and perpendicularly to the above mentioned rotor axis. FIG. 4 also shows a side and another front view of the compressor along with the average human figure in order to visualize a representative size of the device. Other embodiments of the invention may vary in size and proportions and may place the entrance and exit connections for gas in other orientations such as, for example, both coaxial with the rotor axis or both lateral, etc.

Innovative technical characteristics of the present device include:

1. It uses an electric, synchronous, axial flow motor with permanent magnets as driving force mounted on the same axis as the impellers of the centrifugal compressor. This type of motor is more efficient and has higher power density than high speed radial flow motors, which gives this device a superior global performance and a smaller physical size compared to the current art.

2. It uses passive magnetic bearings and passive electrodynamic bearings that do not require any energy supply, auxiliary system or monitoring or control system. This characteristic gives the device a high operating reliability, even in case of sudden electric supply fault. Additionally, the absence of control auxiliary systems contributes to its compact size.

3. It does not use mechanical seals since the rotor assembly is placed totally inside the same pressure containment as the process gas. The mechanical seals suffer from wear by friction and require frequent maintenance, especially in high speed applications. Its absence gives this device the feature that it requires less maintenance than other prior art equipment. Additionally, the absence of mechanical seals contributes to the global energy efficiency of the equipment.

4. It does not use any kind of lubricants for seals, gears or bearings. This characteristic contributes to the low maintenance requirement of the equipment and also to its reduced size, since there is no need of auxiliary systems for treatment of lubricant, such as coolers, filters, separators, or pumps.

5. Under normal conditions, due to the novel contact-free rotating support system, the assembly rotates at the same speed as the impeller of the compressor without suffering any mechanic wear. 

What is claimed is:
 1. A compact gas compressing device comprising a rotating motor-impeller assembly formed by one or more centrifugal compressor impellers (1) and an electric motor (2), in which the one or more compressor impellers are coupled directly and on a single axis (5) to the electric motor (2), wherein said electric motor (2) is a synchronous, axial flow and permanent magnet motor.
 2. The compact gas compression device in accordance with claim 1, wherein the axis (5) of the rotating motor-impeller assembly is supported by two or more magnetic radial bearings (6, 7) to fix the radial position of the axis (5), and one or more passive electrodynamic thrust bearings (8, 9) to fix the axial position of the axis, and wherein the magnetic radial bearings (6, 7) and the one or more electrodynamic thrust bearings (8, 9) operate totally free of lubricants and of auxiliary control systems.
 3. The compact gas compression device in accordance with claim 1, wherein each one of said one or more electric motors (2) are formed by one or more stator assemblies (4) located between one or more rotating assemblies (3) which are fixed to the axis (5); and wherein the stator assemblies (4) contain one or more coils (10) and the rotating assemblies (3) contain one or more pairs of permanent magnets (12).
 4. The compact gas compression device in accordance with claim 4, wherein the one or more stator assemblies (4) further comprise part of the ferromagnetic core (11).
 5. The compact gas compression device in accordance with claim 3, wherein some of the one or more rotating assemblies (3) have ferromagnetic cores (11).
 6. The compact gas compression device in accordance with claim 3, wherein said one or more coils receive current pulses activated by a control electronic device (19) which monitors the position of magnets (12),
 7. The compact gas compression device in accordance with claim 3, wherein said control device (19) comprises semiconductors of the group including, among others: mosfet, IGBT, SSR.
 8. The compact gas compression device in accordance with claim 2, wherein each one of the magnetic radial bearings (6, 7) is formed by a rotating section comprising one or more permanent magnets with ring geometry fixed to the axis (5) and a stator section also formed by one or more permanent magnets with ring or cylinder geometry and that circumferentially surround the rotating section; and wherein both sections are separated by an elastic force of magnetic repulsion.
 9. The compact gas compression device in accordance with claim 2, wherein each one of said electrodynamic thrust bearings (8, 9) is formed by a rotating section fixed to the axis (5) and formed by two or more discs (8) that contain permanent magnets and ferromagnetic cores; and a static section fixed to the housing of the device and formed by a solid or perforated conducting disc (9) which is located between both rotating discs (8) and which comprises the conducting material; and wherein the relative movement between the rotating discs (8) and said conducting material induces electrical currents that generate repulsion forces against said magnets.
 10. The compact gas compression device in accordance with claim 2, wherein each one of said electrodynamic thrust bearings is formed by a static section fixed to housing of the device and formed by two or more discs that contain permanent magnets and ferromagnetic cores; and a rotating section fixed to the rotating axis of the device and formed by a solid or perforated conducting disc placed between said static discs and comprising conducting material; and wherein the relative movement between static discs and said conducting material induces electric currents thereon that generate repulsion forces against said magnets.
 11. The compact gas compression device in accordance with claim 1 or 2, wherein said rotating motor-impeller assembly, said magnetic radial bearings (6, 7) and said electrodynamic thrust bearings (9, 10) are placed inside the pressure containment of process gas (16), in a totally water tight container which is free of mechanical seals.
 12. The compact gas compression device in accordance with claim 1 or 2, wherein the same process gas is used as a coolant for said electric motor (2) and said magnetic radial bearings (6, 7) and said electrodynamic thrust bearings (8, 9).
 13. The compact gas compression device in accordance with claim 1, wherein the same process gas is used as a coolant for the power electronics (19) driving the electric motor.
 14. The compact gas compression device in accordance with claim 1, wherein the device is free of auxiliary systems for cooling, filtration, separation or feeding of any kind of lubricants.
 15. The compact gas compression device in accordance with claim 1, wherein the device comprises a compressor impeller (1) and an electric motor (2).
 16. The compact gas compression device in accordance with claim 1, wherein the device comprises two or more compressor impellers (1) and one electric motor (2).
 17. The compact gas compression device in accordance with claim 2, wherein the motor-impeller rotating assembly is mounted on an axis (5) supported by two magnetic radial bearings (6, 7) and a passive electrodynamic thrust bearing (9, 10).
 18. The compact gas compression device in accordance with claim 2, wherein the motor-impeller rotating assembly is mounted on an axis (5) supported by two magnetic radial bearings (6, 7) and two or more passive electrodynamic thrust bearings (9, 10).
 19. The compact gas compression device in accordance with claim 2, wherein the motor-impeller rotating assembly is mounted on an axis (5) supported by more than two magnetic radial bearings (6, 7) and two or more passive electrodynamic thrust bearings (9, 10). 